1. Field of the Invention
The present invention relates generally to pneumatic tires, and in particular to a pneumatic tire being ideal as a studless tire and having both of on-ice performance and on-snow performance improved in a well balance while suppressing occurrence of uneven wear.
2. Description of Related Art
It is known that a studless tire for an icy or snowy road, which is characterized by a tread pattern with five block rows that comprises a central block row, a pair of middle block rows on the outside thereof and a pair of shoulder block rows being provided outside the middle block rows as disclosed in Japanese unexamined Published Applications NOS. 2005-280367 and 2006-298057. If the number of block rows is not less than 6, it is difficult to keep steering stability on a dry road because of lack of pattern rigidity since the blocks are too small. If the numbers of block rows is not more than 4, it is difficult to keep on-snow performance because of lucking shearing force against snow under the tread. In such a tread pattern with the five block rows, shapes and numbers of the sipes in each block, ratio of the land portion to the tread pattern, and groove depths and the like have been determined in order to improve on-ice and on-snow performances.
In the recent years, with the development of speeding up and upgrading of a car, further improvement of the on-ice and on-snow performance is anticipated.